Fix crash during binary cache fetches when using legacy module cache

tuist generate --binary-cache no longer crashes mid-fetch with a Task created in a session that has been invalidated error when TUIST_LEGACY_MODULE_CACHE=true is set. The crash was caused by the shared URLSession being invalidated repeatedly while downloads were still in flight. HTTPSettings now only invalidates the shared session when the network configuration actually changes, so repeated config loads during generation leave active cache downloads intact.